The present invention relates generally to optical methods of interconnecting electrical circuits to avoid the problems associated with electrical transmission line interconnections. More particularly, the invention concerns techniques for interconnecting printed circuit boards or modules in a parallel and broadcast topology an optical waveguide reflective star coupler array.
High speed multi-board digital processing system require inter-board interconnections. Many of these systems require a backplane bus interconnection topology consisting of many parallel data, clock and control signal lines. The operation of these high speed digital systems requires short clock cycle times in addition to high data rates. Systems with short clock cycle times require short, deterministic and precise propagation delays within the backplane interconnect. This is difficult to achieve without careful consideration of electrical transmission line propagation effects.
Currently for the interconnection of high speed digital circuit boards, impedance matched transmission lines are used. To design a continuous impedance transmission line it is necessary to utilize a continuous reference ground line or plane. For a bus topology on a backplane this continuity is difficult to acheive at all loading conditions, usually resulting in a compromise between reflections on the transmission line and packaging design complexity. Reflections on the backplane control lines cause undesired signal edges which are detrimental to systems that use edge triggered events. Reflections on the backplane data lines make it necessary for the system's designer to include a settling time so that the reflections will subside before the data is sent. This settling time must be added to the propagation delay of the transmission line and thus adds to the overall bus delay.
The propagation delay of the transmission line depends upon the distributed capacitance per unit length of the transmission line and distributed load capacitance per unit length. The fully loaded bus condition will usually result in the worst case propagation delay. For good system reliability, the system designer must consider this worst case propagation delay. As a consequence, there is a decrease in system performance under light load conditions. The load capacitances of the transceivers and connectors at each board also decrease the impedance of the bus and increase the propagation delay. Since in many systems the number of boards can be 8 to 32 or more, the resulting distributed load capacitance can be very high compared to the transmission line capacitance per unit length. The result is increased power consumption to drive a high fan out bus with the propagation delay being dependent upon the load and impedance matching conditions of the bus transmission line.
Other limitations of electrical busses are poor reliability due to the high pin count connectors required. A limitation with parallel transmission lines on the backplane is that the lines must be physically separated proportional to the bandwidth of the signals being propagated and the length that the lines are parallel to avoid crosstalk. With fast rising digital pulses the effective bandwidth of the signal is higher, resulting in additional crosstalk. Techniques such as digital trapezoidal waveforms have been implemented to minimize crosstalk problems, but at the expense of reduced signal bandwidth.
Optical methods for interconnecting circuits have been investigated in an effort to avoid the foregoing problems associated with the design of electrical transmission line interconnections. Optical interconnections have been explored because optical signals are immune from EMI effects, can provide dense interconnections with less crosstalk, and are free from loading effects on the propagation delay through the interconnection media.
In general, the topology required for the interconnection of a multi-board processing system is a parallel set of linear tapped broadcast busses. The number of parallel busses varies and is dependent upon the bus protocol and required data transfer rate of the system. By utilizing the bandwidth of an optical system and incorporating time-division and wavelength division multiplexing the required number of parallel busses can be reduced by an order of magnitude relative to the electrical transmission line methods. However, one major constraint of optical waveguide technology to the application of backplane interconnections is the lack of developed optical tapped couplers to implement linear tapped busses. Several methods to efficiently tap optical power from a multi-mode "trunk" fiber are known. However, the performance of these coupler is either mode population dependent, fiber type or geometry dependent, or labor intensive to fabricate.
For the optimum linear tapped bus performance, the tapped coupler must be bidirectional and asymmetric. In addition, the tapped powder ratio must be adjustable during the fabrication process of the tapped coupler and achieve low coupler excess loss. These required characteristics are difficult to simultaneously achieve in one coupler design. If all of these characteristics are not available in one coupler design, then compromises to the optimum linear tapped topology must be made. These compromises are either increased propagation delay, clock distribution skew, or increased receiver dynamic range and/or number of connections per signal line.
Transmissive star couplers have also been inventigated as a means for achieving a parallel broadcast topology for application to a backplane. The transmissive star coupler approach suffers from physical implementation limitations such as fiber band radius and requires separate transmit and receive fibers for each signal passed.